[SDK: Python] Add Webhook endpoints/interface

[infinityplusone]

Changes

Adds comprehensive Webhooks v2 client support to the tango-python SDK.

Added

• Event type discovery: list_webhook_event_types() to discover supported event types and subject types
• Subscription management: Full CRUD operations for webhook subscriptions
  • list_webhook_subscriptions() - List subscriptions with tier-capped pagination (page + page_size)
  • get_webhook_subscription() - Get a single subscription by ID
  • create_webhook_subscription() - Create a new subscription
  • update_webhook_subscription() - Update subscription name and/or payload
  • delete_webhook_subscription() - Delete a subscription
• Endpoint management: Full CRUD operations for webhook endpoints
  • list_webhook_endpoints() - List endpoints with standard pagination
  • get_webhook_endpoint() - Get a single endpoint by ID
  • create_webhook_endpoint() - Create a new endpoint (one per user)
  • update_webhook_endpoint() - Update endpoint callback URL and/or active status
  • delete_webhook_endpoint() - Delete an endpoint
• Testing utilities:
  • test_webhook_delivery() - Send a test webhook to verify endpoint connectivity
  • get_webhook_sample_payload() - Fetch sample payloads for development/testing
• Type definitions: Complete dataclasses for all webhook models (WebhookSubscription, WebhookEndpoint, WebhookEventType, etc.)

Changed

• HTTP client: Added _patch() and _delete() helper methods for webhook management endpoints

Why

This implements the full Webhooks v2 API surface area from the Tango server, enabling SDK users to:

• Discover available event types and subject types
• Manage webhook subscriptions with proper tier-based access controls
• Manage webhook endpoints (though typically provisioned by MakeGov in production)
• Test webhook delivery and inspect sample payloads during development

Testing

• Comprehensive test suite covering all webhook endpoints
• Tests verify correct HTTP methods, request bodies, query parameters, and response parsing
• All tests pass with mocked HTTP responses

Notes

• Pagination: Subscriptions use page + page_size (tier-capped), while endpoints use page + limit
• Tier gating: "Catch-all" subscriptions (empty subject_ids) require Enterprise tier
• Deliveries/redelivery: The server API does not currently expose public endpoints for listing deliveries or redelivery operations
• Signature verification: Documentation includes Python code snippet for verifying webhook signatures (HMAC-SHA256)

Related

Resolves makegov/tango#1274